MRI is an effective, non-invasive technique for generating sharp images of the internal anatomy of the human body, which provides an efficient means for diagnosing disorders such as neurological and cardiac abnormalities and for spotting tumors and the like. Briefly, the patient is placed within the center of a large superconducting magnetic that generates a powerful static magnetic field. The static magnetic field causes protons within tissues of the body to align with an axis of the static field. A pulsed radio-frequency (RF) magnetic field is then applied causing the protons to begin to process around the axis of the static field. Pulsed gradient magnetic fields are then applied to cause the protons within selected locations of the body to emit RF signals, which are detected by sensors of the MRI system. Based on the RF signals emitted by the protons, the MRI system then generates a precise image of the selected locations of the body, typically image slices of organs of interest.
A significant problem with MRI is that its strong magnetic fields can interfere with the operation of any medical devices, particularly pacemakers or ICDs, implanted within the patient. Typically, pacemakers and ICDs include pulse generators for generating electrical pacing pulses and shocking circuits for generating stronger defibrillation shocks. A set of conductive leads connect the pulse generators and shocking circuits to electrodes implanted within the heart. An individual pacing pulse is applied by using the pulse generators to generate a voltage difference between a pair of the electrodes, typically between a tip electrode implanted within the right ventricle and the pacemaker housing or “can.” A defibrillation shock is applied by using the shocking circuits to generate a much larger voltage difference between a pair of the electrodes, typically between a coil electrode implanted within the right ventricle and the pacemaker housing. The pulse generators, shocking circuits, leads and electrodes, as well as the tissue and fluids between the electrodes, collectively provide a conduction loop. During normal pacing operation of the device, current is permitted to flow around the conduction loop. The precise normal loop is from the pacing output circuitry through the pacing lead conductor to the lead tip, then into the right ventricular muscle, and then into the chest skeletal muscles and then back to the can and thence to the inner circuitry again. Care must be taken to ensure that defibrillation shocks do not induce currents within the pacing conduction loops, which might damage the pacing circuitry. This includes internal as well as external defibrillation shocks. Diodes and the like are used to prevent such damage.
State of the art pacemakers and ICDs exploit lead systems having numerous electrodes, thus presenting numerous possible conduction paths. Accordingly, diodes are provided along all vulnerable current pathways. Under normal operating conditions, these safeguards are helpful. However, the powerful magnetic fields of an MRI system can abuse these protection components and induce currents to flow around the pacing current paths by generating unwanted voltage differentials between various electrode pairs. The resulting currents and voltages can have severe consequences to the patient. In particular, the pulsed gradient components of the MRI can include currents among the conduction paths sufficient to trigger unwanted pulses or shocks. These are referred to as parasitic currents. The resulting rapid pulses could, in certain cases, induce life-threatening fibrillation of the heart. The RF fields are not a problem as such fields do not stimulate cardiac cells and are blocked by bypass capacitors in the pacemaker or ICD and hence do not enter the device.
Another significant concern is that the induced voltages are sensed by the pacemaker as heartbeats. In most pacing modes, such as VVI or DDI, the pacemaker then assumes that the heart needs no help and will then block its pacing output (i.e. delivery of a pacing pulse is inhibited.) This could cause a “pacing dependent” patient to pass out or die. VVI and DDI are standard device codes that identify the mode of operation of the device. Others standard modes include DDD, VDD and VOO. Briefly, DDD indicates a device that senses and paces in both the atria and the ventricles and is capable of both triggering and inhibiting functions based upon events sensed in the atria and the ventricles. VDD indicates a device that sensed in both chambers but only paces in the ventricle. A sensed event on the atrial channel triggers a ventricular output after a programmable delay. VVI indicates that the device is capable of pacing and sensing only in the ventricles and is only capable of inhibiting the functions based upon events sensed in the ventricles. DDI is identical to DDD except that the device is only capable of inhibiting functions based upon sensed events, rather than triggering functions. As such, the DDI mode is a non-tracking mode precluding its triggering ventricular outputs in response to sensed atrial events. VOO identifies fixed-rate ventricular pacing, which ignores any potentially sensed cardiac signals. This mode is quite different from the aforementioned “demand” modes, which only pace when the pacemaker determines that the heart is “demanding” pacing. Numerous other device modes of operation are possible, each represented by standard abbreviations of this type.
Various methods have been proposed to address the effects of interference by MRI systems on implantable medical devices. Typically, such safeguard techniques operate to detect the strong fields associated with an MRI and then switch sensing modes or pacing modes in response thereto. See, for example, U.S. Patent Application 2003/0083570 to Cho et al.; U.S. Patent Application 2003/0144704 to Terry et al.; U.S. Patent Application 2003/0144705 to Funke; U.S. Patent Application 2003/0144706 also to Funke and U.S. Pat. No. 6,795,730 to Connelly, et al., entitled “MRI-Resistant Implantable Device.”
Heretofore, however, it does not appear that the typical techniques for safeguarding implantable medical devices from MRI fields properly distinguish among the different types of magnetic fields generated during MRI. Typical techniques merely monitor for a strong magnetic field and, if one is detected, safeguard procedures are then initiated by the implanted device. However, the effects on the patient and on the implanted device can vary depending upon the particular field being applied. One technique that at least addresses the distinction among the different types of MRI fields is set forth in U.S. Patent Application 2004/0088012 of Kroll et al., entitled “Implantable Stimulation Device With Isolating System For Minimizing Magnetic Induction,” which is incorporated by reference herein. Techniques are described therein that employ separate magnetic field sensors and RF signal sensors. In one example, safeguard procedures are activated only if both sensors detect strong fields, thus reducing risks of false MRI detection. In other examples, safeguard procedures are activated if either a strong magnetic field or a strong RF signal is detected. However, room for improvement remains. In particular, the aforementioned technique of Kroll et al. does not distinguish between static magnetic fields and pulsating gradient magnetic fields. The static magnetic field of an MRI system typically does not induce parasitic currents, whereas the pulsating gradient components can induce such currents. Accordingly, it would be highly desirable to provide improved MRI safeguarding techniques that distinguish between static magnetic fields and pulsating gradient magnetic fields and it is to this end that the invention is generally directed.